This invention relates to a tandem master cylinder for hydraulic brake systems of automotive vehicles of the type having central valves in the push-rod piston and in the floating piston. Preferably, the central valve in the push-rod piston is coupled to the floating piston. The gasket of the tandem master cylinder and the central valve in the floating piston can be tested for leakage by pressurizing the pressure chamber of the push-rod piston with a pressure fluid which is supplied through the connection to the reservoir where the pressure fluid is stored.
German Published Patent Application (Deutsche Offenlegungsschrift) No. 36 29 564 describes a brake system having a slip control arrangement consisting of a pedal-operated braking pressure generator which is preferably auxiliary-energy-assisted. The wheel brakes are connected to the braking pressure generator via pressure lines of a hydraulic auxiliary pressure supply system which also includes a hydraulic pump, a pressure-compensation-and-pressure-fluid storage reservoir, with an auxiliary pressure control valve, and wheel sensors and electronic circuits for detecting the rotational behavior of the wheels and for generating electric braking pressure control signals which control electromagnetically operable pressure fluid inlet valves and outlet valves. The inlet valves and the outlet valves, located in the pressure fluid line effect slip control. The brake system of German Published Patent Application No. 36 29 564 is characterized by a switch which is located into the pressure line of the auxiliary energy source and operates after a predetermined hydraulic pressure has been reached to emit an electric signal to the signal processing and control circuit. This, in turn, causes the motor of the pump to switch on and off for the purpose of developing a checking cycle which signals the driver of the vehicle via a visual or acoustic indication that the pump-and-motor unit is ready for operation.
U.S. Pat. Application Ser. No. 367,156, filed June 16, 1989 and entitled "Method for Assembling a Piston-and-Cylinder Unit with a Force Transmission Element" corresponds to German Patent Application No. P 38 23 630.3 and describes a method for assembling a piston-and-cylinder unit with a force transmission element such as a pressure element. The unit has, at least, a pressure piston with a valve element which opens or closes the pressure chamber associated with the piston depending on the movement of the pressure piston. U.S. Patent Application Ser. No. 367,156 suggests that, in order to reduce the lost travel of the spring-loaded closure member and/or of the spring-loaded valve seat of the valve element, the pressure piston be moved against a spring force with a force to be measured which undergoes a marked increase due to the closing operation of the valve element and that the travel of the piston from its initial position up to the marked increase in force be used for selecting one or a plurality of spacers to be positioned between the force transmission element and the piston in order to reduce the lost travel.
Prior art tandem master cylinders have been arranged to permit conducting the so-called probe test. This test involves the pressurization of the pressure chamber associated with the push-rod piston to test the gaskets of the tandem master cylinder for leakage and to test the central valve in the floating piston of the tandem master cylinder for leakage, with the tandem master cylinder mounted on the vehicle. This test, for example, is performed by the automobile manufacturer.
The following steps are carried out in the probe test. With the tandem master cylinder mounted in place on the vehicle, a test probe is pressed onto the storage reservoir return bore of the push-rod piston. Pressure fluid is forced at a high pressure into the pressure chamber of the push-rod piston through the return bore and the hydraulic passages downstream from the return bore. As a result of the pressure in the push-rod piston, the floating piston will move toward the bottom of the tandem master cylinder, while the push-rod piston will remain still. A problem with prior art tandem master cylinders has been that, during the probe test, the closure member of the central valve of the push-rod piston is pulled out of the push-rod piston. This can cause damage to, or a complete failure of, the tandem master cylinder, so that proper operation of the tandem master cylinder cannot be assured.